MR enhancing layer (MREL) for spintronic devices

ABSTRACT

The performance of an MR device has been improved by inserting one or more Magneto-Resistance Enhancing Layers (MRELs) into approximately the center of one or more of the active layers (such as AP1, SIL, FGL, and Free layers). An MREL is a layer of a low band gap, high electron mobility semiconductor such as ZnO or a semimetal such as Bi.

FIELD OF THE INVENTION

The invention relates to the general field of magneto-resistance (MR) devices with particular reference to improving their performance.

BACKGROUND OF THE INVENTION

A typical example of a spintronic device is a magneto-resistance sensor. As shown schematically in FIG. 1, this includes seed layer 11, an antiferromagnetic (AFM) layer 12, outer pinned layer 13 (anti-parallel 2 or AP2), AFM coupling layer 14, typically, but not limited to, Ru, inner pinned layer 15 (AP1), spacer layer 16, free layer (FL) 17, and capping layer 18. Depending on whether spacer layer 16 is conductive or insulating (i.e. a barrier layer), the sensor is either a Giant Magneto-Resistance (GMR) device or a Tunneling Magneto-Resistance (TMR) device.

As recording densities increase, device sizes become correspondingly smaller. Consequently the sensor (or other spintronic device) has to continue having a high enough magneto-resistance (MR) ratio, as well as a low enough resistance.area product (R.A), for it to still have sufficient output amplitude and also for it to be suitable for future extendibility.

It is, however, becoming increasingly more difficult for MgO-based TMR devices to maintain their current high MR ratio in the lower R.A region. Current Perpendicular to the Plane (CPP) GMR or Confined Current Path (CCP) GMR devices have not yet been able to produce high enough MR ratios for them to take over as the future read head technologies. There is thus an urgent need for sensors that have higher output amplitude and/or a higher M.R ratio.

The present invention, as will be detailed below, discloses some novel methods for further enhancing the MR ratio.

A routine search of the prior art was performed with the following references of interest being found:

U.S. Pat. No. 7,596,015 (Kitigawa et al) shows a free layer including an intermediate layer of Cu and/or Zn or Sn. (See col. 5, lines 58-63 free layer is the same as recording layer 12, and see col. 11, lines 1-15 for materials added to the recording layer—at least one of the materials including Cu, Zn, and Sn).

U.S. Pat. No. 7,223,485 (Yuasa et al) teaches an insertion layer in the free layer. (See col. 2-3 where the insert layer contains at least one element from Cu, Zn, O). K. Zhang et al, HT06-055 “a novel CPP device with enhanced MR ratio” and the reference therein. Y. Chen et al, “Spin-dependent CPP transport properties of ZnO/ferromagnet Heterostructures” Physics Letters A 303 (2002) 91-96

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the present invention to provide a method for enhancing the performance, particularly the MR and R.A ratios, of magneto-resistance devices.

Another object of at least one embodiment of the present invention has been to provide a process for manufacturing the invention

A further object of at least one embodiment of the present invention has been that said process be fully compatible with existing processes for manufacturing spintronic devices.

Still another object of at least one embodiment of the present invention has been to provide a detailed description thereof.

These objects have been achieved by inserting one or more Magneto-Resistance Enhancing Layers (MRELs) into approximately the center of one or more of the active layers (such as the AP1, SIL, and Free layers). An MREL is a layer of a low band gap, high electron mobility semiconductor such as ZnO or a semimetal such as Bi. Optionally, the interface(s) between the MREL and the active layer into which it has been inserted may be bridged by a thin layer of a highly conductive metal such as copper so as to ensure an ohmic contact across the full interface between the MREL and the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical MR device of the prior art.

FIG. 2. Perpendicular RH curve for a Ta10/Ru20/Cu20/[Co2/Ni6]x15/Cu20/FeCo150/Ru10/Ta40/Ru30 structure of the prior art.

FIG. 3. Perpendicular RH curve for the structure shown in FIG. 2 modified to become Ta10/ . . . x15/Cu20/FeCo75/Cu3/ZnO15/Cu3/FeCo75/ . . . /Ta40/Ru30.

FIG. 4 Cross-sectional view of a typical MREL

FIG. 5 illustrates a typical MR device after insertion of MRELs in both the AP1 and free layers.

FIG. 6 shows the three main component sections of a spin torque oscillator (STO), including the FGL into which has been inserted an MREL.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General We have found that when a trilayer of a low band gap semiconductor such as ZnO, sandwiched between layers of a conductive metal such as Cu (to form Cu3/ZnO15/Cu3 for example), is inserted into a spintronic device the MR ratio of that device is increased significantly. An early example of this was discovered in the course of experiments with a spin torque oscillator (STO), patterned to around 45×45 nm, whose structure was: Ta10/Ru20/Cu20/[Co2/Ni6]x15/Cu20/FeCo150/Ru10/Ta40/Ru30. This device had almost no MR ratio for the in-plane RH measurement. However, when the applied field was close to the perpendicular direction (specifically, H was tilted 7 deg from the perpendicular direction), the observed MR ratio was typically about 1.5% with an R.A of about 0.06. A typical RH curve is shown in FIG. 2.

After insertion of the above-specified trilayer the resulting full structure became: Ta10/Ru20/Cu20/[Co2/Ni6]x15/Cu20/FeCo75/Cu3/ZnO15/Cu3/FeCo75/Ru10/Ta40/Ru30).

Its MR ratio was found to have increased significantly (from about 1.3% to as high as 17%) with little change in the R.A product. A typical curve is shown in FIG. 3. This measurement was performed at an angle of 7 degrees off vertical so it is likely that if the applied field used for the measurement is made to be precisely normal to the plane of the film, a much higher MR value will be obtained.

In FIG. 4 we show a schematic cross-section of a MREL in which central semiconductor (or semimetal) layer 41 is seen to be sandwiched between two (optional) metal layers 42 and 43. Some additional findings about the MREL structure were:

First, the thickness chosen for the Cu3/ZnO15/Cu3 trilayer must ensure that the two 75 Angstrom FeCo layers on either side of it are strongly exchange coupled with each other. The fulfillment of this condition was confirmed when a MH loop measured on a Ta20/Ru20/IrMn70/FeCo75/Cu3/ZnO15/Cu3/FeCo75 film stack showed only one hysteresis loop for the two FeCo layers, indicating that said layers are strongly coupled together.

Second, an in-plane measurement showed almost no MR ratio, which is very similar to what was observed for the reference case. These facts indicate that the high MR ratio cannot be coming from the magnetic switching of the [FeCo75/Cu3/ZnO15/Cu3/FeCo75], but rather from the [Co2/Ni6]x15/Cu20/[FeCo75/Cu3/ZnO15/Cu3/FeCo75] switching across the Cu spacer. Thus the [FeCo75/Cu3/ZnO15/Cu3/FeCo75] behaves as though it is a single layer (like the original FeCo150 layer that it replaced.

This confirms that the Cu3/ZnO15/Cu3 trilayer acts as an MR enhancing layer. It is thought that the spin scattering coefficient of the [FeCo75/Cu3/ZnO/Cu3/FeCo75] is significantly enhanced by the insertion of the Cu3/ZnO15/Cu3 trilayer. This is in accordance with the Schmitt theory, which shows that when spin is injected from a FM metal (FeCo) into a semiconductor (e.g. ZnO), the spin coefficient is proportional to the ratio of the conductivity of the semiconductor to that of the FM metal.

It is also possible that the Cu3/ZnO/Cu3 acts as a spin filtering layer, possibly even one operating under a ballistic scattering regime. Regardless of the precise mechanism at work here, the MR enhancement that results from inserting the Cu/ZnO/Cu (or similar layer) can be realized in a wide variety of applications as we will detail below.

Before describing several applications for these MRELs we note the following:

In the general case, the MREL's structure is M1/low band gap, high mobility, N-type semi-conductor/M2, where

(a) M1 and M2 are high conductivity metals such as (but not limited to) Cu, Ag, Au, C (including Graphene, nano-tubes etc), Zn, Ti, Sn, Cr, Al, Mg, and Ru. M1 and M2 will generally be the same material and will have the same thickness (ranging from 0 to 50 Å), with about 3 Å Cu for both M1 and M2 being preferred. However, M1 and M2 may be different materials and they may have different thicknesses without significantly affecting the effectiveness of the invention.

(b) Additionally, the basic M1/semiconductor/M2 trilayer structure that makes up the MREL may be repeated in situ one or more times, each such additional trilayer not necessarily comprising the same materials and/or individual thicknesses as other trilayers in the same MREL.

(c) The band gap of the semiconductor portion of the MREL should be in a range of from 0.3 to 8 eV, with from 1 to 6 eV being preferred.

(d) The electron mobility of the semiconductor portion of the MREL should be in a range of from 10 to 2,000,000_cm²·sec⁻¹·V⁻¹ with from 50 to 50,000 cm²·sec⁻¹·V⁻¹ being preferred.

Examples of suitable semi-conductor materials include (but are not limited to) ZnO, ZnS, Zn_(x)Mg_((1-x))O (x ranging from 0 to 0.99), ZnCuO, ZnCdO, ZnAlO, ZnSe, ZnTe, Si, Ge, TiO₂, AlN, GaN, InN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, ZnS, CdS, CdTe, HgTe, PbS, PbSe, PbTe, SnO, SnTe, Cu₂O, FeSi₂, CrMnSi, Mg₂Si, RuSi₃, Ir₃Si₅ etc.

The semiconductor portion may be undoped or its conductivity may be adjusted by the inclusion of a dopant selected from the group consisting of Si, B, Mg, Mn, Al, Cu, Cd, Cr, Zn, Ti, Sn, Zr, Hf, Ru, Mo, Nb, Co, Fe, and Ni.

The semiconductor portion may be replaced by a semi-metal such as: Sb, Bi, CoSi, Co_(x)Fe_((1-x))Si, Co_(x)Ni_((1-x))Si, Co_(x)Mn_((1-x))Si, FeSi, or Co_(x)Cr_((1-x))Si.

The semiconductor/semimetal layer should have a thickness in a range of from 1 to 50 Å, with from 3 to 20 Å being preferred.

We now describe several examples of how these MR enhancing layers may be used to improve the performance of existing MR devices. These include (but are not limited to):

GMR devices (including CIP, CPP, and CPP/CCP versions), TMR devices, spin injection devices, magnetic random access memories (MRAMs) and microwave assisted magnetic recording (MAMR) devices (see item 4 below).

It will be understood that those skilled in the art may be able to identify additional spintronic devices not listed above (such as, for example, sensors of all types (including biosensors), thermally assisted magneto-resistance (TAMR) devices or dual spin valve devices) to which the principles disclosed by the present invention would obviously be applicable.

1. GMR Devices:

Prior art structure is: seed/AFM/AP2/Ru/AP1/conductive spacer/FL/cap.

Possible structures based on the invention (see FIG. 4):

seed/AFM/AP2/Ru/[FM1/MREL/FM2]/cond. spacer/FL/cap [enhanced AP1]

seed/AFM/AP2/Ru/AP1/cond.spacer/[FL1/MREL/FL2]/cap [enhanced free layer]

seed/AFM/AP2/Ru/[FM1/MREL/FM2]/cond. spacer/[FL1/MREL/FL2]/cap

The spacer layer could be a uniform metallic layer (CPP devices) or it could be in the form of conductive islands embedded in an insulating matrix such as, for example, Cu in Al₂O₃ (CPP/CCP devices).

2. TMR Devices:

Prior art structure is: seed/AFM/AP2/Ru/AP1/barrier layer/FL/cap.

Possible structures based on the invention (see FIG. 4):

seed/AFM/AP2/Ru/[FM1/MREL/FM2]/barrier layer/FL/cap [enhanced AP1]

seed/AFM/AP2/Ru/AP1/barrier layer/[FL1/MREL/FL2]/cap [enhanced free layer]

seed/AFM/AP2/Ru/[FM1/MREL/FM2]/barrier layer/[FL1/MREL/FL2]/cap

The barrier layer could be MgO, AlO, TiO, ZnO, Zn/ZnO, or Zn/ZnO/Zn (as a laminate or as ZnO with a stoichiometric excess of Zn).

FIG. 5 is a schematic view of a GMR/TMR device after insertion of MRELs into both the AP1 layer (split into sub-layers 15A and 15B) and the free layer (split into sub-layers 17A and 17B).

3. Spin Injection Layer (SIL) Devices that Operate Without an AFM/AP2/Ru/AP1 Pinning Stack:

Prior art structure is: seed/FM/spacer/FL/cap (including an H field normal to the layers)

Possible structures based on the invention (see FIG. 5):

seed/[FM1/MREL/FM2]/spacer/FL/cap

seed/FM/spacer/[FL1/MREL/FL2]/cap]

seed/FM/[FM1/MREL/FM2]/spacer/[FL1/MREL/FL2]/cap

4. Spin Torque Oscillator (STO) Devices (Particularly the FGL or Field Generating Layer):

Prior art structure is: seed/SIL/spacer/FGL/cap (including an H field normal to the layers)

Possible structures based on the invention (see FIG. 5):

seed/[FM1/MREL/FM2]/spacer/FGL/cap

seed/SIL/spacer/[FL1/MREL/FL2]/cap]

seed/[FM1/MREL/FM2]/spacer/[FL1/MREL/FL2]/cap

FIG. 6 shows the three main component sections of a spin torque oscillator (STO), including SIL 62, inter-layer 63, and the FGL comprising the sub-layers 64A and 64B which are separated from one another by MREL 51.

For the examples listed in the above four categories, the seed layer could be Ta/Ru, Ta/Cu, Ta/NiFe, Ta/NiCr, Ta/NiFeCr, Ta/Ti, Ta/Ti/Cu, Ta/Ti/Ru/Cu, etc, the AFM layer (when present) could be IrMn, PtMn, PtPdMn etc, and AP2 (where present) could be CoFe, CoFe/FeTa/CoFe, CoFe/CoFeB/CoFe etc. 

What is claimed is:
 1. A method to enhance performance of a spintronic device having at least one ferromagnetic layer, comprising: inserting within one or more of said ferromagnetic (FM) layers a magneto-resistance enhancing layer (MREL) comprising a semimetal or semiconductor (S) formed between two conductive layers M1, M2 to give a FM1/M1/S/M2/FM2 configuration where FM1 is a first portion of the at least one FM layer that contacts a lower surface of the MREL, and FM2 is a second portion of the FM layer that contacts an upper surface of the MREL, and M1 and M2 are selected from a group consisting of Cu, Ag, Au, C (including graphene and nano-tubes), Zn, Ti, Sn, Cr, Al, Mg, and Ru.
 2. The method recited in claim 1 wherein said at least one FM layer is selected from the group consisting of magnetically pinnable layers, magnetically free layers, spin injecting layers and field generating layers.
 3. The method recited in claim 1 wherein said semiconductors have band gaps in a range of from 1 to 6 eV and electron mobilities in a range of from 50 to 50,000 cm²·sec⁻¹·V⁻¹.
 4. The method recited in claim 3 wherein said semiconductors are selected from the group consisting of ZnO, ZnS, Zn_(x)Mg_((1-x))O, ZnCuO, ZnCdO, ZnAlO, ZnSe, ZnTe, Si, Ge, TiO₂, AlN, GaN, InN, AlP, AIAs, AISb, GaP, GaAs, GaSb, InP, InAs, ZnS, CdS, CdTe, HgTe, PbS, PbSe, PbTe, SnO, SnTe, Cu₂O, FeSi₂, CrMnSi, Mg₂Si, RuSi₃, and Ir₃Si₅.
 5. The method recited in claim 4 wherein said semiconductors are undoped or their conductivity is adjusted by inclusion of a dopant selected from the group consisting of Si, B, Mg, Mn, Al, Cu, Cd, Cr, Zn, Ti, Sn, Zr, Hf, Ru, Mo, Nb, Co, Fe, and Ni.
 6. The method recited in claim 1 wherein said semimetals are selected from the group consisting of Sb, Bi, CoSi, Co_(x)Fe_((1-x))Si, Co_(x)Ni_((1-x))Si, Co_(x)Mn_((1-x))Si, FeSi, and Co_(x)Cr_((1-x))Si.
 7. The method recited in claim 1 wherein each of the conductive layers M1, M2 has a thickness from about 0.1 to 50 Angstroms.
 8. The method recited in claim 1 wherein the semiconductor or semimetal layer (S) has a thickness from about 1 to 50 Angstroms.
 9. An enhanced performance spintronic device having a spacer layer formed between two ferromagnetic layers (FM and FL), comprising: the ferromagnetic (FM and FL) layers selected from the group consisting of magnetically pinnable layers, magnetically free layers, spin injecting layers and field generating layers; and in addition to said spacer layer, a magneto-resistance enhancing layer (MREL), having upper and lower surfaces, and comprising an n-type semiconductor (S) selected from the group consisting of semiconductors and semimetals and located entirely within one or both of said FM and FL layers to give a FM1/MREL/FM2 and/or FL1/MREL/FL2 configuration where FM1 and FL1 are a first portion of the FM and FL layers, respectively, that contacts a lower surface of an adjoining MREL, and FM2 and FL2 are a second portion of the FM and FL layers, respectively, that contacts an upper surface of the adjoining MREL.
 10. The enhanced performance spintronic device described in claim 9 wherein one or both of the FM and FL layers further comprises a first conductive layer (M1) that contacts an upper surface of the FM1 and/or FL1 layer, and a second conductive layer (M2) that contacts a bottom surface of the FM2 and/or FL2 layer to give a FM1/M1/S/M2/FM2 or FL1/M1/S/M2/FM2 configuration.
 11. The enhanced performance spintronic device described in claim 10 wherein the M1 and M2 conductive layers are selected from the group consisting of Cu, Ag, Au, C (including graphene and nano-tubes), Zn, Ti, Sn, Cr, Al, Mg, and Ru.
 12. The enhanced performance spintronic device described in claim 9 wherein said semiconductors have band gaps in a range of from 1 to 6 eV and electron mobilities in a range of from 50 to 50,000 cm²·sec⁻¹·V⁻¹.
 13. The enhanced performance spintronic device described in claim 9 wherein said semiconductors are selected from the group consisting of ZnO, ZnS, Zn_(x)Mg_((1-x))O, ZnCuO, ZnCdO, ZnAlO, ZnSe, ZnTe, Si, Ge, TiO₂, AlN, GaN, InN, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, ZnS, CdS, CdTe, HgTe, PbS, PbSe, PbTe, SnO, SnTe, Cu₂O, FeSi₂, CrMnSi, Mg₂Si, RuSi₃, and Ir₃Si₅.
 14. The enhanced performance spintronic device described in claim 13 wherein said semiconductors are undoped or their conductivity is adjusted through inclusion of a dopant selected from the group consisting of Si, B, Mg, Mn, Al, Cu, Cd, Cr, Zn, Ti, Sn, Zr, Hf, Ru, Mo, Nb, Co, Fe, and Ni.
 15. The enhanced performance spintronic device described in claim 9 wherein said semimetals are selected from the group consisting of Sb, Bi, CoSi, Co_(x)Fe_((1-x))Si, FeSi, and Co_(x)Cr_((1-x))Si. 